Halon

[Robert_A_Brady@xxxxxxxxxxxx (Robert A Brady)]

Halon extinguishes flames by scavanging the Hydrogen radicals.  For 
combustion to continue (propagate), hydrogen radicals are required to split 
the Oxygen molecules forming an Oxygen radical and a hydroxy radical.  

Halogens such as Bromine and Florine are more reactive with the hydrogen 
radicals than the O2 is, so they wind up scavanging the hydrogen and thus 
combustion can't propagate.

CO2 and dry chemical extinguishers, on the other hand, work by displacing the 
oxygen from the fuel.  Water extinguishers work primarilly by cooling the 
reactants which slows the propagation reactions down to the point where they 
terminate before they can continue.  Metal fires don't depend on hydrogen 
radicals to propagate, and therefore Halon is worthless on them.  I'm not 
sure how you put out a metal fire (i.e. what's in the metal fire 
extinguishers), but I'd suspect they are some inert ingredient that displaces 
the oxygen from the fuel.  

Bob Brady, DVAROC
Recovering Combustion Research Engineer

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